Stable ocean platform

ABSTRACT

A stable, floating marine platform is provided having a natural period of oscillation greater than the period of the waves of maximum energy to which the platform may be exposed. A plurality of vertically and radially extending damping plates are circumferentially spaced around the upper and lower submerged portions of the platform, and a horizontal damping plate is secured to the bottom of the platform to prevent resonance oscillation of the platform.

United States Patent Johnson, Jr. et a1.

STABLE OCEAN PLATFORM lnventors: Virgil E. Johnson, .Ir., Gaithersburg; John 0. Scherer, ,lr., Laurel; Eugene R. Miller, Annapolis; Viggo A. Blues, Pikesville, all of Md.

Assignee: Hydronauties, lnc., Laurel, Md.

Filed: July 30,1969

Appl. No.: 870,874

Related US. Application Data Division of Ser. No. 745,285, July 16; 1968, Pat. No. 3,500,783.

U.S.Cl. ..137/799, 137/236, 138/106, 138/118, 138/178 Int. Cl ..Fl6l 11/00 Field ofSearch ..l37/236,6l5, 799; 138/178, 138/118; 114/.5, 230; 9/8; 166/.5; 175/5; 61/.5,

References Clted UNITED STATES PATENTS 2/1939 Feykert ..137/799 x 51 Jan. 18,1972

3,360,810 1/1968 Busking ..9/8 OP 1,933,597 11/1933 McVitty..... 114/230X 2,955,626 10/1960 Hartley ..l37/236X 3,111,69211/1963 Cox 137/236X 3,111,926 11/1963 Shatto.... 114/230X 3,221,816 12/1965 Shatto ..166/.5 3,359,741 12/1967 Nelson... 61/72.3X 3,372,409 3/1968 Manning.... l37/236X 3,376,708 4/1968 l-lindman... ..166/.5X 3,407,768 10/1968 Graham ..114/230X 3,487,858 1/1970 Hanback ..137/178X Primary Examiner-Harold W. Weakley Attorney-Finnegan, Henderson & Farabow [57] ABSTRACT A stable, floating marine platform is provided having a natural period of oscillation greater than the period of the waves of maximum energy to which the platform may be exposed. A plurality of vertically and radially extending damping plates are circumferentially spaced around the upper and lower submerged portions of the platform, and a horizontal damping plate is secured to the bottom of the platform to prevent resonance oscillation of the platform.

7 Claims, 6 Drawing lllgures FIG PATENIEllJuwm 3.635.253

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INVMORS vmsn. zmaonusou, JOHN o. ERER, Eussus .mLLEmm. VIGGO A. BLAES Elwyn/z, lna asolz QIZLMow ATTORNEYS STABLE OCEAN PLATFORM This application is a division of application Ser. No. 745,285, filed July 16, 1968, now Pat. No. 3,500,783.

A stabilized, flow line system is provided for maintaining fluid connection between a stable, floating marine platform and an underwater wellhead under all sea conditions. The system includes an upper, substantially vertically disposed flow line and a lower, substantially horizontally disposed flow line that are connected in angular relationship, said connection being moored so as to maintain the respective substantially vertical and horizontal orientation of the flow lines. The connection is also moored to permit it to rise and fall with the heaving of the marine platform.

This invention relates to stable ocean platforms suitable for oil well drilling, petroleum production and storage, communication facilities, satellite observations, and the like. More particularly, this invention relates to an inherently stabilized floating marine platform for offshore petroleum production and storage, and a flow line system for the platform that remains intact with an underwater wellhead under all sea conditions.

As the search for oil extends further offshore into deeper and deeper water, it becomes increasingly difficult tovprovide bottom-supported platforms to conduct the necessary drilling and production operations. The cost of such platforms not only are prohibitive, but their safety is questionable.

Floating platforms have more recently been used in attempts to avoid the disadvantages inherent in bottom-supported platforms. For such platforms to be capable of carrying out the drilling, oil production, and other offshore operations, they must be relatively stable despite the motions of the sea. By stable, as used throughout the specification and claims, is meant a platform having a minimum of translational and angular motion so that it is habitable at all times and that all necessary operations can be carried out under even the most severe sea conditions.

Floating facilities that have been provided in the past, however, have not proven entirely satisfactory. These facilities, for example, generally have low stability and the fluid connection to wellhead is susceptible to rupture even under moderate sea conditions and especially under resonance conditions. Further, many of the platforms and the mooring systems heretofore provided in attempts to improve the stability and wellhead connection have either been too complex or too costly to be practical.

Accordingly, a primary object of this invention is to provide a new and improved flow line system for connecting a floating marine platform to a wellhead, which system is not subject to rupture despite the motions of the platform under the most severe sea conditions.

In accordance with this invention, a stabilized, flow line system is provided for connecting a floating marine platform to an underwater wellhead, which system comprises a lower, flexible fluid flow line substantially horizontally disposed while being maintained clear of the ocean floor and extending upwardly from the wellhead, and an upper, flexible fluidflow line substantially vertically disposed and extending downwardly from the platform.

In accordance with the invention, means are provided for connecting the upper end of the lower flow line to the lower end of the upper flow line in angular relationship, and mooring means are provided for maintaining the connection at a point laterally spaced from the platform to maintain the flow lines in their respective substantially vertical and horizontal orientation. The mooring means is also sufficiently flexible to permit the connection to rise and fall with heaving of the marine platform. Preferably, the upper flow line is connected to the platform through a coil of flow line that has a degree of flexure sufficient to accommodate angular motion of the platform relative to the flow line.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory but are not restrictive of the invention.

The accompanying drawings which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and together with the description, serve to explain the principles of the invention.

0f the drawings:

FIG. 1 is a perspective view showing the mooring and flow line system;

FIG. 2 is a detail taken within the circle 5 of FIG. 1 and showing the connection between the upper and lower flow lines when the underwater wellhead is laterally spaced a distance from the platform;

FIG. 3 is a view similar to FIG. 2 and showing the connection between the upper and lower flow lines when the underwater wellhead is substantially beneath the platform;

FIG. 4 is a detail of a flexible flow line coil for connecting theupper flow line to the platform;

FIG. 5 is a view similar to FIG. 4 and showing an alternative flow line coil; and

FIG. 6 is a detail showing a method for constructing the flow line coil of FIGS. 4 and 5.

Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings.

For the purposes of clarity and simplification, only those parts of a floating marine platform which are essential to an understanding of the stabilizing and flow line system of this invention have been illustrated in the drawings.

The floating marine platfonn of this invention, generally indicated as 20, is an elongated, right circular cylinder having a hull 22 of substantially constant cross-sectional area and a production and crew facility area 24. Hull 22 is internally closed at each end and is divided into a plurality of compartmented petroleum storage tanks. The storage tanks can be filled with either sea water or petroleum, the petroleum displacing the sea water as it is pumped into the tanks.

As shown in FIG. 1, platform 20 is conventionally moored at the desired location with a plurality of chain or steel rope mooring lines 28. Mooring lines 28 are attached to the platform sufficiently below the water line of the hull to prevent them from being damaged by vessels: approaching the platform. The lines are anchored to the ocean floor at 30 and are of a length sufficient to define a ca'tenary curve from the ocean floor to the platform. Preferably, the mooring lines are circumferentially spaced around the platform but their exact location may vary depending upon the existence of prevailing ocean currents, as is well known to those skilled in the art.

Since the lines are in a form of a catenary curve, they are continuously in tension and, therefore, lateral movements of the platform caused by wave action, wind, and/or current, are resisted by the tension in the lines. The spring tension of the lines, however, is selected so that the expected lateral movements or surge of the platform can occur without placing undue stress on the mooring system. The mooring lines, therefore, are not used to prevent lateral oscillation but to prevent drifting of the platform away from the desired location.

In the open sea, the period or wavelength of expected oscillatory waves vary considerably from short periods to very long periods of over 50 seconds. Generally, however, ocean waves are irregular, and while a continuous spectrum of all wave periods is therefore present at any one time, certain wave periods are predominant depending upon the conditions of the sea. Since waves at the predominant period are the most numerous, these waves have the highest energy, and hence the greatest influence on the movement of a floating body. Under sea state 5, for example, the period of waves of the highest energy is about l0 seconds, while under sea state 8, the period of waves of the highest energy is around 25 seconds.

As the oscillatory waves strike a floating body, various motions are imparted to the body and the motions increase with increasing wave energy. Typical of the motions imparted to a floating body by oscillatory waves are heaving (vertical motion), pitching (angular motion), and surging (lateral motion).

Considering the response of a floating body, it can be seen that if the natural period of oscillation of a floating body is at or near the period of the waves of highest energy striking the platform at any given time resonance will occur and motions become so violent that the platform will be totally useless. Thus, it is desirable to provide a platform in which the natural period of oscillation of the body is considerably in excess of the period of the waves of maximum energy to which a floating marine platform may be exposed in the open sea. The period of waves of maximum energy is generally around 22 seconds, which are the predominant wave periods under hurricane conditions.

In accordance with this invention, there is provided a stabilized, flow line system for connecting a floating marine platform to a wellhead on the bottom of the ocean floor. As embodied, and as shown in FIG. 1, this system comprises a lower, flexible fluid flow line 50 substantially horizontally disposed and extending upwardly from a wellhead 52 and an upper, flexible fluid flow line 54 substantially vertically disposed and extending downwardly from platform 20.

The system also includes flow line connecting means for connecting the upper end of lower flow line 50 to the lower end of upper flow line 54 in angular relationship. As embodied and as shown in FIG. 2, the connecting means comprises a rigid elbow 56 that connects the upper and lower flow lines in obtuse angular relationship and above the ocean floor.

Mooring means are further provided to hold the connecting means at a location laterally spaced from the platform to maintain the upper and lower flow lines in their respective substantially vertical and horizontal orientation. As embodied, this means comprises a pair of mooring lines 58 extending in a substantially horizontal direction away from lower flow line 50 and suitably anchored to the ocean floor at 60 and a bridle 62. The ends of lines 58 are attached to elbow 56 at the junction between the elbow and the respective flow lines. The two (2) ends of bridle 62 are proportioned so that they meet the flow lines tangentially to eliminate bending moments in the flow lines arising from tension on mooring lines 58. The purpose of mooring lines 58 is to keep flow lines 50 and 54 under sufficient tension so that they are maintained in their respective substantially horizontal and vertical orientation, but flexible enough to allow for vertical and lateral movement of the platform in response to wave action. In addition, the mooring means restrains the flow lines against the forces arising from currents in the sea.

As shown in FIG. 1 where wellhead 52 is spaced laterally a significant distance from platform 20, elbow 56 connects upper and lower flow lines 54 and 50, respectively, in such a manner that an obtuse angle is formed between the lines. Alternatively, and as shown in FIG. 3, when the wellhead is located substantially beneath the platform, elbow 56' is constructed so that an acute angle is formed between the upper and lower flow lines. In this embodiment, mooring lines 58 extend in an opposite direction from the wellhead to maintain tension in the flow lines in a manner similar to the embodiment shown in FIGS. 1' and 2.

While only a single flow line system has been illustrated in the drawings, it is to be understood that a plurality of flow lines may be constructed in accordance with this invention and incorporated with a single floating platform.

To provide for pitching motion of the platform relative to the flow lines, flexible connecting means are provided for connecting upper flow line 54 to the platform. As embodied and as shown in FIG. 4, the flexible connecting means comprises a helical coil 70 of flow line having a substantially horizontal axis and a degree of flexure greater than the maximum expected angular movement of platform relative to flow line 54. One end of helical coil 70 is connected to hull 22 of platform 20 at 72 which, in turn, is connected to the pumps (not shown) and the other end of coil 70 through connection 74 to flow line 54. As platform 20 pitches in response to wave action, coil 70 flexesand thereby prevents rupture between the pipe line and the platform. The invention thus provides a In the preferred embodiment of this invention and to relievetension on coil 70 from the weight of the flow line system, a tension link 76 is provided having its lower end secured to connection 74 and its upper end pivotally connected at 80 to hull 22. The link may be of any suitable form, such as a chain cable or bar. Link 76 pennits connection 74 between coil 70 and flow line 54 to swing through an arc about point 80 and accommodate angular motion of platform 22 relative to flow line 54 while continuously supporting the flow line and maintaining the connection between it and coil 70.

Alternatively, the helical coil of flow line may be carried externally of the hull, the coil 70' being slightly modified, as shown in FIG. 5. The upper end of flow line 54 is similarly connected to hull 22 by tension link 76' to relieve tension on coil 70.

In either of the embodiments shown in FIGS. 4 and 5, thecoil of flow line can be constructed either from a continuous length of tubing rolled in the form of a helix or as shown in FIG. 6, it may be constructed from a plurality of pipe elbows interconnected at 92 to form a continuous coil 70.

By way of example, and without any intention to be limited thereto, a 7-inch diameter flow line would require approximately a 3-turn coil 70 having a diameter of about 50 feet to accommodate the angular motion of the platform relative to the flow line.

Thus it can be seen that the invention provides a new and improved stable, floating marine platform that remains habitable and operational under all sea conditions, especially resonance conditions, and a flow line system that remains intact between the platform and underwater wellhead despite the conditions of the sea.

This invention in its broader aspects is not limited to the specific details shown and described and departures may be made from such details without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed is:

l. A stabilized flow line system for connecting a floating marine platform to an underwater wellhead comprising a lower, flexible fluid flow line substantially horizontally disposed and extending upwardly from a wellhead, an upper, flexible flow line substantially vertically disposed and extending downwardly from a platform, flow line connecting means for connecting the upper end of said lower flow line to the lower end of said upper flow line in angular relationship, and mooring means for maintaining the connecting means at a point laterally spaced from a platform and the upper and lower fluid flow lines in their respective substantially vertical and horizontal orientation, wherein the mooring means includes a bridle secured to the connecting means and a plurality of mooring lines secured between the bridle and the ocean floor and extending in a substantially horizontal direction away from the lower flow line.

2. The system of claim 1, wherein the wellhead is located laterally a significant distance from the platform, and the connecting means forms an obtuse angle between flow lines.

3. The system of claim I, wherein the wellhead is located substantially underneath the platform and the connecting means forms an acute angle between the flow lines.

4. The device of claim I, wherein the connecting means comprises an elbow section of flow line.

5. The device of claim 1, including a helical coil of flow line having a substantially horizontal axis, and means for connecting one end of the coil to the platfomi and the other end to the upper flow line, said coil having a degree of flexure greater than the maximum expected angular movement of the platform in response to wave action relative to the flow line.

6. The device of claim 5, including a tension link for pivotally connecting the upper flow line to the platform.

"I. The device of claim 1 wherein the bridle includes a first end coupled tangentially to said lower fluid flow line and a second end coupled tangentially to said upper fluid flow line to eliminate bending movements in the flow lines arising from tension on said mooring means. 5

i =l: it l l 

1. A stabilized flow line system for connecting a floating marine platform to an underwater wellhead comprising a lower, flexible fluid flow line substantially horizontally disposed and extending upwardly from a wellhead, an upper, flexible flow line substantially vertically disposed and extending downwardly from a platform, flow line connecting means for connecting the upper end of said lower flow line to the lower end of said upper flow line in angular relationship, and mooring means for maintaining the connecting means at a point laterally spaced from a platform and the upper and lower fluid flow lines in their respective substantially vertical and horizontal orientation, wherein the mooring means includes a bridle secured to the connecting means and a plurality of mooring lines secured between the bridle and the ocean floor and extending in a substantially horizontal direction away from the lower flow line.
 2. The system of claim 1, wherein the wellhead is located laterally a significant distance from the platform, and the connecting means forms an obtuse angle between flow lines.
 3. The system of claim 1, wherein the wellhead is located substantially underneath the platform and the connecting means forms an acute angle between the flow lines.
 4. The device of claim 1, wherein the connecting means comprises an elbow section of flow line.
 5. The device of claim 1, including a helical coil of flow line having a substantially horizontal axis, and means for connecting one end of the coil to the platform and the other end to the upper flow line, said coil having a degree of flexure greater than the maximum expected angular movement of the platform in response to wave action relative to the flow line.
 6. The device of claim 5, including a tension link for pivotally connecting the upper flow line to the platform.
 7. The device of claim 1 wherein the bridle includes a first end coupled tangentially to said lower fluid flow line and a second end coupled tangentially to said upper fluid flow line to eliminate bending movements in the flow lines arising from tension on said mooring means. 